In general, to meet industry requirements, a ballast for a gas discharge lamp has to meet the following fundamental requirements: (i) draw power from a power line a power factor of at least 90%, (ii) draw current from the power line with total harmonic distortion of less than 20%, and (iii) provide a load current crest factor of less than 1.7, where the "crest factor" is equal to a peak magnitude of the load current divided by its effective (RMS) value.
Furthermore, there are many other requirements like: a) circuit simplicity--parts count and cost shall be low, b) the design shall be very adaptable to all line voltages and lamps kinds as well as number of lamps to be powered, c) the design shall be easy manufacturable in large quantities with great repeatability as required by industry quality standards, d) the design shall be insensitive to high and low ambient temperatures, e) the design shall operate within specification when smaller power rated lamps of the same group being connected as load, f) the design shall be insensitive to line voltage variations, and g) the design shall provide for long life of a ballast. These requirements and more all apply to electronic ballasts as well.
In an electronic ballast, which generally requires a well filtered DC voltage for operating an inverter that powers the gas discharge load, a front stage converter is required. Then, such well filtered DC voltage may be attained without incurring any substantial harmonic distortion of the current drawn from the power line. However, the cost penalty associated with the front end converter in an electronic ballast is substantial. It is highly desirable to have a simple and low cost single stage electronic ballast which will meet all of the above requirements.
Many known devices of this kind have been described previously, notably, in the following U.S Pat. Nos.: 4,017,785 to Perper; 4,109,307 to Knoll; 4,782,268 to Fahnrich, and 5,010,277 to Courier de Mere.
Construction and evaluation of the above inventions, revealed, that some devices meet many of the requirements mentioned above, however, none of the devices satisfies all of them.